From fdd68dcbe67fac0d8b035497e19e6d9f2c1e888a Mon Sep 17 00:00:00 2001
From: Jon Ross-Perkins <jperkins@google.com>
Date: Tue, 22 Jul 2025 15:31:41 -0700
Subject: [PATCH] Fix a crash when Core is poisoned (#5838)

There are probably other ways to reproduce this, but this is roughly how
I ran into it.
---
 toolchain/check/import.cpp                    |  2 +-
 .../packages/core_name_poisoning.carbon       | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/toolchain/check/import.cpp b/toolchain/check/import.cpp
index 8cba932493ed..7ea3b914fafd 100644
--- a/toolchain/check/import.cpp
+++ b/toolchain/check/import.cpp
@@ -648,7 +648,7 @@ auto ImportNameFromOtherPackage(
 
     const auto* import_scope_entry = LookupNameInImport(
         *import_ir.sem_ir, import_scope_id, name_id, identifier);
-    if (!import_scope_entry) {
+    if (!import_scope_entry || !import_scope_entry->result.is_found()) {
       continue;
     }
     SemIR::InstId import_scope_inst_id =
diff --git a/toolchain/check/testdata/packages/core_name_poisoning.carbon b/toolchain/check/testdata/packages/core_name_poisoning.carbon
index 22346a1ba742..ac3010dba7f7 100644
--- a/toolchain/check/testdata/packages/core_name_poisoning.carbon
+++ b/toolchain/check/testdata/packages/core_name_poisoning.carbon
@@ -12,6 +12,8 @@
 
 // --- fail_implicitly_poison_core.carbon
 
+library "[[@TEST_NAME]]";
+
 // CHECK:STDERR: fail_implicitly_poison_core.carbon:[[@LINE+4]]:9: error: `Core.Bool` implicitly referenced here, but package `Core` not found [CoreNotFound]
 // CHECK:STDERR: fn F(x: bool);
 // CHECK:STDERR:         ^~~~
@@ -19,3 +21,27 @@
 fn F(x: bool);
 
 class r#Core {}
+
+// --- fail_poisoned_core.carbon
+
+package Core library "[[@TEST_NAME]]";
+
+// This introduces a poisoned name in `Core`.
+// CHECK:STDERR: fail_poisoned_core.carbon:[[@LINE+4]]:8: error: name `Core.Int` implicitly referenced here, but not found [CoreNameNotFound]
+// CHECK:STDERR: var x: i32;
+// CHECK:STDERR:        ^~~
+// CHECK:STDERR:
+var x: i32;
+
+// --- fail_use_poisoned_core.carbon
+
+library "[[@TEST_NAME]]";
+
+import Core library "poisoned_core";
+
+// This accesses the poisoned name in `Core`.
+// CHECK:STDERR: fail_use_poisoned_core.carbon:[[@LINE+4]]:9: error: name `Core.Int` implicitly referenced here, but not found [CoreNameNotFound]
+// CHECK:STDERR: fn F(b: i32);
+// CHECK:STDERR:         ^~~
+// CHECK:STDERR:
+fn F(b: i32);